Method and system for non-contact measurement of microwave capacitance of miniature structures of integrated circuits

ABSTRACT

In the method and system for non-contact measurements of microwave capacitance of test structures patterned on wafers used for production of modern integrated circuits, a near-field balanced two-conductor probe is brought into close proximity to a test structure, and the resonant frequency of the probe for the test structure is measured. The probe is then positioned at the same distance from the uniform metallic pad, and the resonance frequency of the probe for the uniform metallic pad is measured. A shear force distance control mechanism maintains the distance between the tip of the probe and the metallic pad equal to the distance between the tip of the probe and the test structure. The microwave capacitance of the test structure is then calculated in accordance with a predefined formula. The obtained microwave capacitance may be further used for determining possible defects of the test structure.

REFERENCE TO RELATED APPLICATIONS

This Utility Patent Application is based on a Provisional PatentApplication Ser. No. 60/428,245 filed 22 Nov. 2002.

FIELD OF THE INVENTION

The present invention relates to measurement techniques. In particular,this invention directs itself to a technique for non-contact measurementof microwave capacitance of miniature structures of integrated circuitsusing near field microwave probes. The concept is based on microwavemeasurements employing a balanced two-conductor transmission lineresonator to obtain a microwave capacitance of miniature structures inorder to monitor integrated circuits fabrication process.

More in particular, the present invention is directed to a method andsystem for non-contact measurement of microwave capacitance of teststructures which do not require knowledge of probe geometry or theabsolute distance from the tip of the probe to the test structure, andin which the microwave capacitance of the test structure is calculatedbased on measured resonant frequencies of the near field microwave probefor the miniature structure under test and the uniform metallic pad. Thedistance between the near field microwave probe and the uniform metallicpad is maintained equal to the distance between the near field microwaveprobe and the test structure by a distance control mechanism during thetesting.

BACKGROUND OF THE INVENTION

A fundamental shift is taking place in the fabrication of integratedcircuits, and in particular in the construction of the multiple wiringlayers that provide connections to the tens of millions of transistorson a state-of-the-art chip. For decades, the standard in the industryhas been the use of aluminum wires isolated from each other by silicondioxide. Despite the success of this combination, it is now placinglimitations on the performance of the chips. In order to obviate theselimitations, the semiconductor industry has begun to search forreplacements for aluminum and/or silicon dioxide that can provideenhanced device performance.

Two shifts in fabrication of integrated circuits are currently takingplace: (1) from aluminum to copper wires to reduce the resistance of themetal wires, and (2) from silicon dioxide to dielectrics with lowerdielectric constants k, commonly referred to as ‘low-k’ materials. Themove away from silicon dioxide as the interlayer dielectric has beendriven by the need to reduce the delay times along the wires in thecomplex circuits. This has opened up a wide field of research anddevelopment in the semiconductor industry focused on the fabrication andcharacterization of new high-performance materials.

However, the stable, well-understood nature of silicon dioxide has ledto an absence of effective tools and methods for characterizingdielectric materials. As a result, progress toward the identificationand optimization of new dielectric materials is slowed by the lack ofcharacterization instruments.

In standard aluminum/silicon dioxide processes, the wiring layers areformed by first laying down a uniform aluminum film, etching away thealuminum in the regions between the wires, and then filling theseregions with silicon dioxide. The switch from aluminum to copper haschanged this process, since there is no effective way to etch copper.Thus, for copper/dielectric based integrated circuits, the Damasceneprocess is generally used, in which the dielectric material (eithersilicon dioxide or a low-k dielectric) is initially deposited. Thedielectric layer is then etched to form trenches where the wires will beformed and finally the copper is deposited into these trenches.

The Damascene process has been somewhat successful, but has also led toa wide range of problems that were not encountered in the standardAl/SiO₂ technology. These problems have been particularly common whenthe typically less-stable low-k dielectrics are used. For example, thedielectric etching process necessary to form the trenches for copperoften damage the dielectric material causing a change in its dielectricconstant, and as a consequence, a change in the performance of thedevice. This type of damage often occurs at the interface between themetal and dielectric and causes changes in the capacitance betweenwires, thus affecting device performance.

Measuring capacitance is an important step in monitoring the fabricationprocess of integrated circuits. The standard method for doing this is tobuild large capacitance test structures directly into the device itselfand then measure the capacitance of these structures by making directcontact to them either through pins on the finished device or by placingsmall probes on contact pads. This type of measurement is typicallylimited to relatively low frequencies (below 1 MHz) and requires largetest structures in order to have a large enough capacitance (>1 picoFarad) to overcome the stray capacitances in the system.

It therefore would be highly desirable to have a non-contact techniquefor capacitance measurement of miniature structures performed atmicrowave (and higher) frequencies.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a technique fornon-contact measurement of microwave capacitance of miniature structuresof integrated circuits performed at microwave (and higher) frequencies.

It is another object of the present invention to perform non-contactmeasurements of the microwave capacitance of test structures with theuse of a near field balanced two-conductor microwave probe. Themeasurement is made at any point in the fabrication process withoutmaking contact with the test structures which may be very small (a fewmicrons or less), and where the measurement is sensitive to extremelysmall changes in capacitance (on the order of 0.1 aF).

It is a further object of the present invention to provide a method andapparatus for measurement of microwave capacitance of miniaturestructures with near field microwave probes which employ an independentdistance control mechanism for maintaining the tip of the probe at anunknown distance from the miniature test structure but nominally equalto the distance of the tip from the uniform metallic pad.

In accordance with the principles herein described, the presentinvention defines a method for non-contact measurement of microwavecapacitance of a miniature structure of integrated circuits, whichincludes the steps of:

-   -   positioning a near field microwave probe at a predetermined        distance from a miniature structure under the test,    -   measuring a resonant frequency f_(s) of said near-field        microwave probe for said miniature structure under the test,    -   positioning said near-field microwave probe at said        predetermined distance from a uniform metallic structure,    -   measuring a resonant frequency f_(c) of said near-field        microwave probe for said uniform metallic structure, and    -   calculating the microwave capacitance C_(s) of said miniature        structure under the test as $\begin{matrix}        {C_{s} = \frac{\left( {f_{e} - f_{c}} \right)\left( {f_{e} - f_{c}} \right)}{\left( {f_{s} - f_{c}} \right)4f_{e}^{2}Z_{0}}} & (1)        \end{matrix}$    -   wherein f_(e) is the resonant frequency of said near-field        microwave probe in air, and Z₀ is the characteristic impedance        of said near-field microwave probe.

The near-field microwave probe is directed to a balanced two-conductortransmission line resonator which includes a pair of conductorsextending in spaced relationship therebetween and separated by adielectric media.

The distance between the near-field microwave probe and the uniformmetallic structure is maintained equal to the predetermined (butunknown) separation between the near-field microwave probe and theminiature structure under the test by a closed-loop distance controlmechanism, which may include a shear force-based or atomic forcedistance control mechanism. The distance maintained is below 50-100 nm.

An area of the uniform metallic structure (contact pad) is preferably atleast as large as the size of a cross-section of near-field microwaveprobe's tip.

The resonant frequencies f_(s) and f_(c) are first obtained by measuringthe absolute value of the first derivative of a power reflected from ortransmitted through the near-field microwave probe as a function of afrequency of a signal applied thereto. Secondly, the resonantfrequencies f_(s) and f_(c) are determined by one of the numericaltechniques, such as:

-   -   (a) determining the resonant frequency f_(s) and f_(c) as the        frequency at the point on said measured curve where said        measured curve has a minimum;    -   (b) determining the resonant frequency f_(s) and f_(c) as the        frequency at the point of said measured curve where the first        derivative of the measured power with respect to frequency        equals zero and second derivative of the measured power is        positive; and/or    -   (c) fitting the obtained measured curve to an even order        polynomial, and finding the frequency where the first derivative        of said polynomial equals to zero.

The miniature structures may include an inter-digital comb-likecapacitor, and/or a single metal wire (or a trench)/dielectric, and/oran array of metal/dielectric trenches, and/or an array of interconnectlines, and/or a multi-layered structure.

The calculated microwave capacitance C_(s) may be further used infabrication quality control by comparing the C_(s) with a predeterminedcapacitance value and judging whether the miniature structure isdefective based on a deviation of the C_(s) from the predeterminedcapacitance value.

The present invention is also a system for non-contact measurement ofmicrowave capacitance of miniature structures of integrated circuits,which includes

-   -   a miniature structure being tested,    -   a near-field microwave probe having a tip,    -   a uniform metallic pad of the size at least equal or larger to        the cross-section of the tip of the near-field microwave probe,    -   a shear force-based (or atomic force based) distance control        unit operatively coupled to the near-field microwave probe to        control tip-to-miniature structure and tip-to-uniform metallic        pad separation,    -   an acquisition mechanism for acquiring resonant frequency f_(s)        of the near-field microwave probe for the miniature structure        under test and the resonant frequency f_(c) of the near-field        microwave probe for the uniform metallic pad, and    -   a processing mechanism for calculating the microwave capacitance        C_(s) of the miniature structure under test as $\begin{matrix}        {C_{s} = \frac{\left( {f_{e} - f_{c}} \right)\left( {f_{e} - f_{c}} \right)}{\left( {f_{s} - f_{c}} \right)4f_{e}^{2}Z_{0}}} & (2)        \end{matrix}$    -   where f_(e) is the resonant frequency of the near-field        microwave probe in air, and Z₀ is the characteristic impedance        of the near-field microwave probe.

In this system, the near-field microwave probe includes a balancedtwo-conductor transmission line resonator having a pair of conductorsextending in spaced relationship therebetween and separated by adielectric media.

The system may further include a mechanism for analysis of thecalculated microwave capacitance C_(s) of the miniature structure undertest for judging whether the miniature structure is defective or not forfabrication quality control purposes.

These and other novel features and advantages of this invention will befully understood from the following detailed description of theaccompanying Drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a balanced two-conductornear-field probe capacitively coupled to an arbitrary test structurewith capacitance C_(s) to be measured;

FIG. 2 schematically depicts a system of the present invention forming atwo-conductor transmission line resonator with a probe end used formeasurement of capacitance of a miniature structure;

FIG. 3 represents schematically the near-field microwave measurements ofthe miniature structure having a pair of wires/trenches; and

FIGS. 4A and 4B represent schematically a measurement scheme of thepresent invention for capacitance measurement of a single wire and asingle trench respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a near-field balanced two-conductor probe 10 isused for quantitative non-contact measurement of the microwavecapacitance of capacitor test structure 12 patterned on a wafer 14 usedfor production of modern integrated circuits. The capacitor teststructure on the wafer can be an inter-digital comb-like capacitor, oran array of metal/dielectric trenches, and/or coupled interconnect lineswhich form multi-layered structures having a single or multiplicity oftrenches/wires embedded in and separated by dielectric 16 of the wafer14. Wafer 14 may have a ground layer 18.

The probe 10, in accordance with the method of the present invention, isbrought into close proximity to the test structure 12 and fixedlypositioned by means of a closed loop distance control mechanism such asshear-force or atomic force based distance control mechanism as will befurther disclosed in detail with regard to FIG. 2.

The near-field probe 10, shown in FIGS. 1-4, used in the method andsystem of the present invention for non-destructive determination of themicrowave capacitance of the test structure 12 is based on a balancedtwo-conductor transmission line 20. As shown in FIG. 2, the balancedtwo-conductor transmission line 20 includes two spatially separated andsymmetrically arranged electrical conductors 22 and 24. The conductors22 and 24 may have a cylindrical, semi-cylindrical, rectangular, orsimilar type cross-section contour. Further, the conductors 22 and 24may be formed out of copper, tungsten, gold, silver, or aluminum stripsdeposited onto a probe dielectric 26, for example, a tapered glassfiber. A probing end (tip) 28 of the transmission line 20 is brought inclose proximity to the test structure 12 and an opposing end 30 of thetransmission line 20 is either connected to electronics, or to aterminating plate 32 to form a resonator structure 34 for purposes to bedescribed in following paragraphs.

The probe 10 may operate as a transmission line for feeding a signal tothe sample 12 and measuring the reflected signal to obtain a moresensitive and accurate result while employing less expensive equipment.However, the probe 10 of the present invention is envisioned as aresonator structure 34 which is formed by a portion of the transmissionline 20 with the conductors 22 and 24 separated by the dielectric medium26. The dielectric medium is any low-loss dielectric which may includeair, a circulating fluid for temperature stabilization, quartz, or highdielectric constant materials for size reduction. As described inprevious paragraphs, the probing end 28 of the resonator structure 34 isbrought into proximity to the test structure 12 with the opposite end 30of the transmission line resonator structure 34 coupled to theterminating plate 32. The resonator structure 34 is formed in order tomeasure the resonant frequency of the resonator structure 34 fordetermination of the microwave capacitance of the test structure 12 aswill be described in detail infra.

The spacing between the two conductors 22 and 24 and their cross-sectionmust be properly chosen for accurate measurement. For instance, thespacing between the conductors 22 and 24 may be on the order of orgreater than 1 mm at the top of the resonator and tapered down to a sizeon the order of the test structure (as small at 50 nm).

The coupling to the resonator 34 is provided by a coupling loop 36positioned close to the resonator 34. The coupling loop 36 is internalto an optional conducting sheath (not shown). An optional secondcoupling loop 38 may be used for measurement electronics 40schematically shown in FIG. 2. All calculations are carried out by dataprocessor 42 based on predetermined formulas applied to the measureddata. The processor 42 additionally controls the overall performance andoperation of the measurement electronics 40 and a distance controlmechanism 44.

The resonator structure 34 forms a (2n+1)λ/4 or (n+1)λ/2 resonator(n=0,1,2, . . . ), and its length is determined by the frequency of thelowest mode, e.g., about 25 mm for the λ/2 mode operating at 4 GHz andquartz dielectric.

The resonator structure 34 may be enclosed in a cylindrical sheathformed of a highly conductive material, such as Cu, Au, Ag, Al). Thesheath eliminates both radiation from the resonator 34 and the effect ofthe probe environment on the resonator characteristics. In particular,the changing influence of moving parts in the proximity of resonator 34is eliminated. Additionally, the sheath has an opening near the teststructure area which allows for an efficient coupling of the teststructure 12 to the resonator 34.

In the situation where the spacing between the conductors 22 and 24 issmall in comparison to the inner diameter of the sheath, the resonatorproperties are substantially unaffected by the sheath's presence. Theupper portion of the sheath makes electrical contact with theterminating plate 32. The bottom portion of the sheath may have aconical shape in order to provide clear physical and visual access tothe sampling area.

As discussed in previous paragraphs, the probing end 28 of the resonatorstructure 34 is brought into close proximity to the test structure 12for measurement purposes. The geometry of the probing end (tip) 28, aswell as separation between the tip 28 and the test structure 12 is vitalto the technique of the present invention for quantitative measurementsof the capacitance of the test structure. Since the accuratedetermination of these parameters is difficult and often impractical,especially for the very small tips (of less than or on the order of afew microns in size), the quantitative measurements of the presentinvention are performed without any knowledge of either the actual tipgeometry or the absolute tip to structure separation. The quantitativemeasurements are made by employing an independent distance controlmechanism 44 schematically shown in FIG. 2 and described in detailinfra.

As shown in FIGS. 1-4, the electrical conductors 22 and 24 of the probe10 are brought into close proximity to the test structure 12. However,they are separated by a small unknown distance, for instance, 50-100 nm.They are capacitively coupled to the structure on the wafer 14 via themicrowave displacement current without making any physical contact.

In general, the probe tip “fringe” capacitance C_(t), can be written asfollows: $\begin{matrix}{\frac{1}{C_{t}} = {\frac{1}{C_{c1}} + \frac{1}{C_{c2}} + \frac{1}{C_{s}}}} & (3)\end{matrix}$where C_(c1) and C_(c2) are the coupling capacitances between the tipconductors 22 and 24 and test structure 12, and C_(s) is the teststructure capacitance to be determined as shown in FIG. 1.

It is desirable to have C_(s)≦min{C_(c1) and C_(c2)}, since in this casethe probe response is going to be mostly determined by the teststructure capacitance. The typical value for the coupling capacitance,C_(c), is in the range of femto-Farads, which allows one to measure teststructures with smaller capacitances. The typical minimum capacitance ofinter-digital comb-like capacitance structures is in the range ofpico-Farads since measurement of smaller values by conventional means isextremely difficult. The method of the present invention is inherentlycapable of measuring much smaller capacitance values.

As an example, the probe 10 includes a foreshortened piece of theuniform transmission line 20 with an open end connected to the tip 28with the “fringe” impedance at the tip 28 given by Z_(t)=1/iωC_(t).Since the tip capacitance C_(t)<<ε₀λ₀, where ε₀ is the permittivity ofvacuum, and λ₀ is the operating wavelength, the structure forms nearly aquarter-wavelength (λ/₄) resonator 34. Using the standard impedancetransformation technique, the following resonant condition for the probe10 is found: $\begin{matrix}{C_{t} = {\frac{f_{e} - f_{res}}{f_{e}}\frac{1}{4f_{e}Z_{0}}}} & (4)\end{matrix}$where f_(e) is the probe resonant frequency in air, f_(res) is theresonant frequency with arbitrary sample present, and Z₀ is thecharacteristic impedance of the transmission line 20 that forms theresonator 34.

Generally, the tip geometry and the tip-to-sample separation areunknown. Therefore, in order to perform quantitative measurements of thecapacitor under test a probe calibration is required. The probecalibration is performed in the method of the present invention asfollows:

-   -   first, the probe resonant frequency, f_(s), is measured for the        structure 12 under test at some unknown but small tip-to-sample        distance, typically 10 to 100 nm, and    -   second, the probe 10 is placed above a uniform metallic area 46        (for example, a contact pad), as best shown in FIG. 2, that is        at least as large as the probe tip cross-section. The tip 28 is        held at the same distance above this area 46 as it was above the        test structure 12 by the distance control mechanism 44 and its        resonant frequency, f_(c), is measured again.

Finally, from Eqs. (1) and (2), the capacitance C_(s) of the structureunder test is found as follows: $\begin{matrix}{C_{s} = \frac{\left( {f_{e} - f_{c}} \right)\left( {f_{e} - f_{s}} \right)}{\left( {f_{s} - f_{c}} \right)4f_{e}^{2}Z_{0}}} & (5)\end{matrix}$where Z₀ can be determined from an independent measurement, ananalytical calculation, and/or a numerical simulation.

While Eqs. (4) and (5) were derived for the quarter-wavelength resonatorused in this example, similar expressions may be obtained for anyresonant structure coupled to the probe tip, such as a half-wavelength(λ/2) strip line, cavity, or a lumped element resonator.

The distance control mechanism 44 of the present invention, as shown inFIG. 2, may be an atomic force based distance control mechanism.However, mechanism 44 is preferably a shear-force based distance controlmechanism where the tip 28 of the resonant structure 34 is maintained atan unknown, but nominally the same or equal distance both in themeasuring of the resonant frequency f_(s) of the probe 10 for the teststructure 12 and for the measurement of the resonant frequency f_(c) ofthe probe 10 for the contact pad 46.

In order to perform the quantitative measurements with the near-fieldmicrowave probes shown in FIGS. 1-4, it is important that the separationbetween the probe tip 28 and the structure under the study 12 beprecisely controlled. Without precise control of this distance, changesin the structure capacitance cannot be distinguished from changes in thedistance. To control the distance between the tip 28 and the teststructure 12, the distance control unit 44 shown in FIG. 2, isincorporated into the measurement scheme of the probe 10. The distancecontrol unit 44 is coupled bi-directionally to the data processor 42 fordata exchange and for control over the operation of the distance controlmechanism 44.

The shear force based distance control mechanism 44 is a distancecontrol mechanism applicable for use in near-field scanning opticalmicroscopy (NSOM). The basic concept of the shear force distance controlmechanism is that a probe, specifically the tip 28 thereof, is flexibleand is mounted onto and dithered by a piezoelectric element or a quartztuning-fork oscillator (TFO) with an amplitude from a few nanometersdown to a few angstroms. When the tip of such a probe is brought inclose proximity to the sample surface 12, the amplitude of the tiposcillation is damped by interaction between the tip 28 and the samplesurface 12. The motion of the tip is detected by an optical beamdeflection technique for the piezo element or by aphase-or-amplitude-locked loop for the tuning fork oscillator.

In the measuring technique of the present invention, a housing (notshown) of the microwave probe 10 is attached to the dithering element,piezoelectric, which in turn is supported by a fine piezo stage. Thus,the tip 28 is dithered by the piezoelectric element with an amplituderanging from a few nanometers down to a few angstroms. The motion of thetip 28 is detected by an optical beam deflection unit which includes alaser generating a laser beam directed via the oscillating tip 28 to aphotodetector. As the tip 28 is brought into close proximity to the teststructure 12, the amplitude of the tip oscillation is changed, i.e.,damped, by interaction between the tip 28 and the test structure 12which is detected by the photodetector.

Responsive to the change of the amplitude of the tip oscillator, thephotodetector generates an output which is a signal indicative of thechange in tip-to-sample separation. The signal from the output of thephotodetector is supplied to an input of a lock-in amplifier,responsively generating a control signal. The control signal generatedat an output is indicative of unwanted changes in the separation betweenthe tip 28 and the test structure 12. This control signal is fed to aproportional integral derivative controller which generates a controlsignal which is fed to the fine piezo Z stage for changing the positionthereof in a required direction. The probe attached to the fine piezo Zstage adjusts its position with respect to the test structure in orderto reach a predetermined separation between the tip 28 and the teststructure 12.

The piezo detector, the lock-in amplifier, the proportional integralderivative controller, and the fine piezo Z stage, in combination withthe laser, form a feedback loop which maintains the amplitude of theoscillation of the tip 28 of the probe fixed at a value less than apredetermined maximum amplitude of oscillations, and thus, permitsprecise distance control down to 1 nm.

In performing the quantitative measurement of resonant frequencies f_(s)and f_(c), as described in previous paragraphs, the following proceduresare performed:

-   -   (a) adjust the shear force distance control mechanism 44 in a        manner that is capable of holding the tip 28 at some fixed        distance d above the test structure 12. Generally, the absolute        value of this distance d is not known. However, the distance may        be estimated by measuring the shear force signal as a function        of the tip-to-sample separation by means of the tip 28        approaching the test structure 12 in the open loop circuitry. It        is preferred to maintain the separation d on the order of or        less than one-tenth of the dimensions of the tip 28;    -   (b) measure the resonance frequency f_(s) of the probe 10 for        the test structure 12, and submit the measured f_(s) to the        measurement electronics 40;    -   (c) move the probe 10 towards the contact pad 46 to position the        tip 28 the same distance d above the contact pad 46 (the        distance between the tip 28 and the contact pad 46 is controlled        by the distance control mechanism 44);    -   (d) measure the resonant frequency f_(c) of the probe 10 for the        contact pad 46, and submit the measured f_(c) to the measurement        electronics 40; and    -   (e) output data corresponding to f_(s) and f_(c) to the data        processor 42 for further calculating the capacitance C_(s) of        the test structure 12 in accordance with Eq. (5).

The data processor 42 may also perform operations needed in furtherquality control of the manufactured integrated circuit on the wafer 14.For this purpose, a predetermined capacitance value for the teststructure is provided. The calculated capacitance C_(s) may be comparedin a block 43 (Integrated Circuit Quality Control) to such apredetermined capacitance value, to permit deciding whether the teststructure is defective based on deviation of the calculated capacitanceC_(s) of the test structure from the predetermined capacitance value.

The potential fault that could be detected using the method of thepresent invention may include but is not limited to such defects as Cuvoids, Cu diffusion into the dielectric, dielectric damage atinterfaces, sidewall damages between the metal of the wire (or a trench)and the dielectric surrounding the metal, etc.

Referring to FIGS. 1, 3, and 4, many types of test structures 12 can besubjected to the non-contact measurement technique of the presentinvention. For example, as shown in FIG. 3, the test structure 12includes at least a pair of lines 46 embedded into the dielectric 16 ofthe wafer 14. In this test structure 12, a capactive coupling C_(c)exists between each electrical conductor 22 and 24 and the respectiveline 46. The line ground capacitance C_(LG) exists between each line 46and the ground layer 18, and the line-line capacitance C_(LL) is formedbetween the lines 46. For such a test structure 12, the totalcapacitance at the probe tip 28 C_(TOT) is described by the equation$\begin{matrix}{C_{TOT} = {\frac{C_{c}C_{LL}}{{2C_{LL}} + C_{c}} + \frac{C_{LG}}{2}}} & (6)\end{matrix}$

In order to be sensitive to variations in C_(LL), the followingconditions are to be satisfied:C _(LG)/2≦C _(LL) ≦C _(c)/2  (7)

For this test structure 12 which has at least a pair of trenches/wires,in accordance with the measurement described with regard to FIG. 1,first the resonant frequency of the probe 10 with regard to thestructure 12 is measured, then the resonant frequency of the probe 10with regard to a uniform metallic area is measured, and the totalcapacitance of the structure 12 is calculated in accordance with Eq.(5).

As one of the examples of a geometry of the test structure 12 that wouldbe appropriate for capacitance measurements of the present invention,the distance 48 between the lines 46 would be ˜0.25 μm, the width 50 ofeach line 46 would be ˜3 μm, the depth 52 of each line 46 would be ˜0.5μm, and the distance between the bottom of the lines 46 and the groundlayer 18 would be ˜0.5 μm. For example, the probe 10 would be separatedfrom the structure 12 by the gap of approximately 3 μm, while the widthof the electrical conductors 22 and 24 would be approximately 1 μm. Withthe geometry as described, the following capacitances have been obtainedby the method of the present invention: C_(c)/2=0.5 f_(F), C_(LL)=0.4f_(F), and C_(LG)/2=0.3 f_(F), which satisfies the condition of Eq. (7).The sensitivity to the changes in capacitance is on the order of 10⁻³f_(F).

The method of the present invention is considered also for measuringcapacitances of a single wire 54 or a single trench 56 as shown in FIGS.4A and 4B, respectively.

The method of the present invention may be used for measuring microwavecapacitances of different types of test structures such as for instancearrays of trenches, wires, single trench, single wire, inter-digitalcomb-like capacitor, etc. In these structures, a plurality of types offaults may be detected such as Cu voids, Cu diffusion into dielectric,dielectric damage to interfaces, sidewall damages, etc. by measuring thecapacitance of a test structure and comparing such a calculatedcapacitance to a predetermined capacitance value. By analyzing thedeviation of the measured test structure capacitance from such apredetermined capacitance one may determine whether the structure isdefective and what kind of defect is encountered with the structureunder test.

With the method and system for non-contact measurement of the microwavecapacitance of test structures, the unique features advantageouslypermitted by such a technique of the present invention include themeasurement at any point in the manufacturing process without makingcontact with the test structure. The method and system is applicable tostructures that are small (a few microns or less) as compared to thestandard test structures which are typically hundreds of microns. Themeasurement is sensitive to extremely small changes in capacitance, onthe order of 0.1 aF, that allows a precise determination of thecapacitance of the test structure.

The resonance frequency of f_(s) and f_(c) is determined by means of oneor some combination of the following numerical techniques which include:

-   -   (a) determining the resonant frequency as the frequency at the        point of the measured curve wherein on the fitting curve of the        measured power vs. frequency, the frequency is at minimum;        and/or    -   (b) determining the relative resonant frequency at the frequency        at the point of the measured curve where the first derivative of        the measured power with respect to frequency equals zero and        second derivative of the measured power is positive; and/or    -   (c) fitting the obtained measured curve to an even order (second        or higher) polynomial, and finding the frequency where the first        derivative of this polynomial equals zero.

Although this invention has been described in connection with specificforms and embodiments thereof, it will be appreciated that variousmodifications other than those discussed above may be resorted towithout departing from the spirit or scope of the invention. Forexample, equivalent elements may be substituted for those specificallyshown and described, certain features may be used independently of otherfeatures, and in certain cases, particular locations of elements may bereversed or interposed, all without departing from the spirit or scopeof the invention as defined in the appended Claims.

1. A method for non-contact measurement of microwave capacitance ofminiature structures of integrated circuits, comprising the steps of:positioning a near-field microwave probe at a predetermined distancefrom a miniature structure under the test; measuring a resonantfrequency f_(s) of said near-field microwave probe for said miniaturestructure under the test; positioning said near-field microwave probesaid predetermined distance from a uniform metallic structure; measuringa resonant frequency f_(c) of said near-field microwave probe for saiduniform metallic structure; and, calculating the microwave capacitanceC_(s) of said miniature structure under the test as$C_{s} = \frac{\left( {f_{e} - f_{c}} \right)\left( {f_{e} - f_{c}} \right)}{\left( {f_{s} - f_{c}} \right)4f_{e}^{2}Z_{0}}$wherein f_(e) is the resonant frequency of said near-field microwaveprobe in air, and Z₀ is the characteristic impedance of said near-fieldmicrowave probe.
 2. The method of claim 1, wherein said near-fieldmicrowave probe includes a balanced two-conductor transmission lineresonator.
 3. The method of claim 1, wherein the measurements areconducted at microwave frequencies.
 4. The method of claim 1, whereinsaid near-field microwave probe includes at least a pair of conductorsextending in spaced relationship therebetween and separated by adielectric medium.
 5. The method of claim 1, further comprising thesteps of: maintaining said distance between said near-field microwaveprobe and said uniform metallic structure equal to said predetermineddistance between said near-field microwave probe and said miniaturestructure under the test by a closed-loop distance control mechanism. 6.The method of claim 5, wherein said closed-loop distance controlmechanism includes a shear force-based distance control mechanism. 7.The method of claim 1, wherein said near-field microwave probe includesa tip, wherein an area of said uniform metallic structure is at leastthe size of a cross-section of said tip of said near-field microwaveprobe.
 8. The method of claim 1, wherein said predetermined distance ismaintained below 50-100 nm.
 9. The method of claim 1, further comprisingthe steps of: measuring the absolute value of the first derivative of apower reflected from or transmitted through said near-field microwaveprobe as a function of a frequency of a signal applied thereto, anddetermining said resonant frequencies f_(s) and f_(c) by a numericaltechnique chosen from the group of numerical techniques, consisting of:(a) determining the resonant frequency f_(s) and f_(c) as the frequencyat the point on said measured curve where said measured curve has aminimum; (b) determining the resonant frequency f_(s) and f_(c) as thefrequency at the point of said measured curve where the first derivativeof the measured power with respect to frequency equals zero and secondderivative of the measured power is positive; and (c) fitting theobtained measured curve to an even order polynomial, and finding thefrequency where the first derivative of said polynomial equals to zero.10. The method of claim 1, wherein said uniform metallic structure is acontact pad.
 11. The method of claim 1, wherein said miniature structureincludes an inter-digital capacitor.
 12. The method of claim 1, whereinsaid miniature structure includes a single metal wire or a trench in adielectric.
 13. The method of claim 1, wherein said miniature structureincludes an array of metal/dielectric trenches.
 14. The method of claim1, wherein said miniature structure includes an array of interconnectlines.
 15. The method of claim 1, wherein said miniature structureincludes a multi-layered structure.
 16. The method of claim 1, furthercomprising the steps of: comparing said calculated microwave capacitanceC_(s) with a predetermined capacitance value, and judging whether saidminiature structure is defective based on a deviation of said C_(s) fromsaid predetermined capacitance value.
 17. A system for non-contactmeasurement of microwave capacitance of miniature structures ofintegrated circuits, comprising: a miniature structure under the test, anear-field microwave probe having a tip thereof, a uniform metallic padof the size approximately equal to the cross-section of said tip of saidnear-field microwave probe, a shear force-based distance control unitoperatively coupled to said near-field microwave probe to controltip-to-miniature structure separation and tip-to-uniform metallic padseparation, acquisition means for acquiring resonant frequency f_(s) ofsaid near-field microwave probe for said miniature structure andresonant frequency f_(c) of said near-field microwave probe for saiduniform metallic pad, and processing means for calculating the microwavecapacitance C_(s) of said miniature structure under the test as$C_{s} = \frac{\left( {f_{e} - f_{c}} \right)\left( {f_{e} - f_{c}} \right)}{\left( {f_{s} - f_{c}} \right)4f_{e}^{2}Z_{0}}$wherein f_(e) is the resonant frequency of said near-field microwaveprobe in air, and Z₀ is the characteristic impedance of said near-fieldmicrowave probe.
 18. The system of claim 17, wherein said near-fieldmicrowave probe includes a balanced two-conductor transmission lineresonator.
 19. The system of claim 17, wherein said near-field microwaveprobe includes at least a pair of conductors extending in spacedrelationship therebetween and separated by a dielectric media.
 20. Thesystem of claim 17, further comprising means for comparing saidcalculated microwave capacitance C_(s) of said miniature structure underthe test with a predetermined capacitance value, and for judging whethersaid miniature structure is defective based on deviation of said C_(s)from said predetermined capacitance value.